[css-shadow-parts][css-pseudo] Move various ::part() details to 'part-like pseudo-elements'. #10083

Author: tabatkins

css-pseudo-4/Overview.bs

@@ -1305,27 +1305,47 @@ Tree-Abiding and Part-Like Pseudo-elements</h2>
     parsing/tree-abiding-pseudo-elements.html
   </wpt>
 
+<h3 id=partlike>
+Part-Like Pseudo-Elements</h3>
+
   A subset of [=tree-abiding pseudo-elements=],
-  the <dfn>part-like pseudo-elements</dfn>,
+  the <dfn export lt="part-like|part-like pseudo-element">part-like pseudo-elements</dfn>,
   have slightly stronger requirements:
   they act as if they have a well-defined location in the document tree.
   This enables them to interact with some other platform features
-  as if they were real elements.
-
-  <div class=example>
-    For example, a [=part-like pseudo-element=]
-    can be used in the <{html-global/exportparts}> attribute,
-    to masquerade as a ''::part()''
-    for the component it's in:
-
-    <xmp class=html>
-      <template id=custom-element-template>
-        <p exportparts="::before : preceding-text">
-          You can style my ::before pseudo-element
-          by using ::part(preceding-text), too!
-      </template>
-    </xmp>
-  </div>
+  as if they were real elements
+  (and, in fact, they often <em>are</em> real elements,
+  such as in the case of ''::part()'',
+  which simply aren't accessible in the current tree).
+
+  [=Part-like pseudo-elements=] inherit from their [=originating element=] by default,
+  like a standard [=tree-abiding pseudo-element=],
+  but can declare that they inherit from another element instead
+  (possibly not accessible).
+  (For example, ''::part()'' inherits from
+  the parent of the element it represents in the shadow tree.)
+
+  All pseudo-classes and pseudo-elements
+  are syntactically allowed after a [=part-like pseudo-element=],
+  such as ''x-button::part(label):hover''
+  or ''x-button::part(label)::before'',
+  but some are disallowed from matching:
+
+  * The [=structural pseudo-classes=],
+    '':has()'' pseudo-class,
+    '':scope'' pseudo-class,
+    and '':host''/'':host()''/'':host-context()'' pseudo-classes
+    never match.
+  * ''::part()'' never matches. (Other [=part-like pseudo-elements=] can, however.)
+
+  A [=part-like pseudo-element=] can define itself as representing a real element
+  (possibly not accessible in the current tree).
+  If it does so, all pseudo-classes and pseudo-elements match according to that element,
+  if not disallowed by the previous restrictions.
+  If it does not do so,
+  it must define which pseudo-classes it matches and when
+  (besides the pseudo-classes defined to work on all pseudo-elements,
+  or all [=tree-abiding pseudo-elements=]).
 
   Unless otherwise specified,
   any CSS property that applies to elements

css-shadow-parts-1/Overview.bs

@@ -109,7 +109,16 @@ from the sub-parts that it merely happens to contain,
 it also helps with encapsulation,
 as authors can use ''::part()'' without fear of accidental over-styling.
 
-
+<!-- Big Text: parts
+
+████▌   ███▌  ████▌  █████▌  ███▌
+█▌  █▌ ▐█ ▐█  █▌  █▌   █▌   █▌  █▌
+█▌  █▌ █▌  █▌ █▌  █▌   █▌   █▌
+████▌  █▌  █▌ ████▌    █▌    ███▌
+█▌     █████▌ █▌▐█     █▌       █▌
+█▌     █▌  █▌ █▌ ▐█    █▌   █▌  █▌
+█▌     █▌  █▌ █▌  █▌   █▌    ███▌
+-->
 Exposing a Shadow Element: {#exposing}
 =============================================================
 Elements in a <a>shadow tree</a> may be exported for styling by stylesheets outside the tree
@@ -146,12 +155,25 @@ and changes to the [=part name lists=] and [=forwarded part name lists=] of elem
 			then let |innerRoot| be its shadow root.
 		3. [=calculate the part element map|Calculate=] |innerRoot|'s [=part element map=].
 		4. For each |innerName|/|outerName| in |el|'s [=forwarded part name list=]:
-			1. Let |innerParts| be |innerRoot|'s [=part element map=][|innerName|]
-			2. [=list/Append=] the elements in |innerParts|
-				to |outerRoot|'s [=part element map=][|outerName|]
+            1. If |innerName| is an ident:
+    			1. Let |innerParts| be |innerRoot|'s [=part element map=][|innerName|]
+    			2. [=list/Append=] the elements in |innerParts|
+    				to |outerRoot|'s [=part element map=][|outerName|]
+            2. If |innerName| is a pseudo-element name:
+                1. [=list/Append=] |innerRoot|'s pseudo-element(s) with that name
+                    to |outerRoot|'s [=part element map=][|outerName|].
 </div>
 
+<!-- Big Text: part=""
 
+████▌   ███▌  ████▌  █████▌      ▐█▌ ▐█▌ ▐█▌ ▐█▌
+█▌  █▌ ▐█ ▐█  █▌  █▌   █▌        ▐█▌ ▐█▌ ▐█▌ ▐█▌
+█▌  █▌ █▌  █▌ █▌  █▌   █▌   ████  █   █   █   █
+████▌  █▌  █▌ ████▌    █▌
+█▌     █████▌ █▌▐█     █▌   ████
+█▌     █▌  █▌ █▌ ▐█    █▌
+█▌     █▌  █▌ █▌  █▌   █▌
+-->
 Naming a Shadow Element: the <{html-global/part}> attribute {#part-attr}
 ------------------------------------------------------------------------
 
@@ -179,6 +201,16 @@ not an id or tagname.
 &lt;/script>
 </pre>
 
+<!-- Big Text: exportparts
+
+█████▌ █     █ ████▌   ███▌  ████▌  █████▌ ████▌   ███▌  ████▌  █████▌  ███▌
+█▌      █   █  █▌  █▌ █▌  █▌ █▌  █▌   █▌   █▌  █▌ ▐█ ▐█  █▌  █▌   █▌   █▌  █▌
+█▌       █ █   █▌  █▌ █▌  █▌ █▌  █▌   █▌   █▌  █▌ █▌  █▌ █▌  █▌   █▌   █▌
+████      █    ████▌  █▌  █▌ ████▌    █▌   ████▌  █▌  █▌ ████▌    █▌    ███▌
+█▌       █ █   █▌     █▌  █▌ █▌▐█     █▌   █▌     █████▌ █▌▐█     █▌       █▌
+█▌      █   █  █▌     █▌  █▌ █▌ ▐█    █▌   █▌     █▌  █▌ █▌ ▐█    █▌   █▌  █▌
+█████▌ █     █ █▌      ███▌  █▌  █▌   █▌   █▌     █▌  █▌ █▌  █▌   █▌    ███▌
+-->
 Forwarding a Shadow Element: the <{html-global/exportparts}> attribute {#exportparts-attr}
 ----------------------------------------------------------------------------------
 Any element in a shadow tree can have a <dfn element-attr for=html-global>exportparts</dfn> attribute.
@@ -200,6 +232,12 @@ Each part mapping is one of:
 
 		Note: This is shorthand for <code>ident : ident</code>.
 
+    : <code>::ident : outerIdent</code>
+    :: If <code>::ident</code> is the name of a [=part-like pseudo-element=],
+        adds <code>::ident</code>/<code>outerIdent</code>
+        to el's [=forward part name list=].
+        Otherwise, does nothing.
+
     : anything else
     :: Ignored for error-recovery / future compatibility.
 </dl>
@@ -235,8 +273,40 @@ Note: It's okay to map a sub-part to several names.
 &lt;/script>
 </pre>
 
+<div class=example>
+    For example, a [=part-like pseudo-element=]
+    can be used in the <{html-global/exportparts}> attribute,
+    to masquerade as a ''::part()''
+    for the component it's in:
+
+    <xmp class=html>
+        <template id=custom-element-template>
+            <p exportparts="::before : preceding-text, ::after : following-text">
+                Main text.
+        </template>
+    </xmp>
+
+    An element using that template
+    can use a selector like ''x-component::part(preceding-text)''
+    to target the ''p::before'' pseudo-element in its shadow,
+    so users of the component don't need to know
+    that the preceding text is implemented as a pseudo-element.
+</div>
+
+
+<!-- Big Text: ::part()
+
+ █▌   █▌  ████▌   ███▌  ████▌  █████▌   ██ ██
+███▌ ███▌ █▌  █▌ ▐█ ▐█  █▌  █▌   █▌    █▌   ▐█
+ █▌   █▌  █▌  █▌ █▌  █▌ █▌  █▌   █▌   █▌     ▐█
+          ████▌  █▌  █▌ ████▌    █▌   █▌     ▐█
+ █▌   █▌  █▌     █████▌ █▌▐█     █▌   █▌     ▐█
+███▌ ███▌ █▌     █▌  █▌ █▌ ▐█    █▌    █▌   ▐█
+ █▌   █▌  █▌     █▌  █▌ █▌  █▌   █▌     ██ ██
+-->
+
 Selecting a Shadow Element: the ''::part()'' pseudo-element {#part}
-============================================================================================
+===================================================================
 
 The <dfn selector>::part()</dfn> pseudo-element
 allows you to select elements that have been exposed via a <{html-global/part}> attribute.
@@ -248,18 +318,14 @@ The syntax is:
 
 The ''::part()'' pseudo-element only matches anything
 when the <a>originating element</a> is a <a>shadow host</a>.
-If the <a>originating element's</a> <a>shadow root's</a> <a>part element map</a>
-[=map/contains=] all of the specified <<ident>>s,
-''::part()'' matches the element or elements keyed to that <<ident>>.
-Otherwise, it matches nothing.
 
 <div class="example">
     For example,
     if you have a custom button
     that contains a "label" element that is exposed for styling
     (via <code>part="label"</code>),
     you can select it with
-    ''#the-button::part(label)''.
+    ''x-button::part(label)''.
 </div>
 
 <div class="example">
@@ -277,15 +343,16 @@ Otherwise, it matches nothing.
 	(or ''::part(active tab)'', as order doesn't matter).
 </div>
 
-The ''::part()'' pseudo-element can take additional pseudo-classes after it,
-such as ''x-button::part(label):hover'',
-but never matches the <a>structural pseudo-classes</a>
-or any other pseudo-classes that match based on tree information
-rather than local element information.
+The ''::part()'' pseudo-element is a [=part-like pseudo-element=].
+If the <a>originating element's</a> <a>shadow root's</a> <a>part element map</a>
+[=map/contains=] the specified <<ident>>,
+''::part()'' represents the elements keyed to that ident;
+if multiple idents are provided and the [=part element map=] contains them all,
+it represents the intersection of the elements keyed to each ident.
+Otherwise, it matches nothing.
 
-The ''::part()'' pseudo-element can also take additional pseudo-elements after it,
-such as ''x-button::part(label)::before'',
-but never match additional ''::part()''s.
+''::part()'' pseudo-elements inherit according to their position
+in the [=originating element's=] shadow tree.
 
 <div class=example>
     For example,
@@ -303,6 +370,18 @@ but never match additional ''::part()''s.
     ignoring any other labels.
 </div>
 
+
+<!-- Big Text: CSSOM
+
+ ███▌   ███▌   ███▌   ███▌  █     █
+█▌  █▌ █▌  █▌ █▌  █▌ █▌  █▌ ██   ██
+█▌     █▌     █▌     █▌  █▌ █▌█ █▐█
+█▌      ███▌   ███▌  █▌  █▌ █▌ █ ▐█
+█▌         █▌     █▌ █▌  █▌ █▌   ▐█
+█▌  █▌ █▌  █▌ █▌  █▌ █▌  █▌ █▌   ▐█
+ ███▌   ███▌   ███▌   ███▌  █▌   ▐█
+-->
+
 Extensions to the {{Element}} Interface {#idl}
 ==============================================